During the manufacture of certain types of printed circuits, particularly electroless-plated printed circuits, an insulating surface of a substrate is to be coated with a treating solution. One specific example of such a process, with which the present invention is especially adapted for use, is disclosed in a related copending application of W. M. Beckenbaugh et al. entitled "A Method for Depositing a Metal on a Surface," Ser. No. 967,549, filed Dec. 7, 1978, now abandoned herein incorporated by reference. As disclosed in that application, a substrate such as epoxy-coated steel is first pretreated with reagents such as methyl ethyl ketone, followed by an etchant containing chromic trioxide, sulfuric acid, and phosphoric acid to render the plastic surface receptive to coating with a special sensitizer solution, such as an aqueous solution of copper formate, the disodium salt of 2, 7 anthraquinone disulfonic acid, sorbitol, and butanol. Examples of such treating solutions and process parameters are provided in the copending application.
As described in that application, the pretreated substrate is coated with the sensitizing solution, as by immersion, dip coating, roller coating, curtain coating, spraying or the like to deposit a layer or film of the sensitizer liquid on the working surfaces of the substrate, that is, the surfaces where printed circuits are to be formed. The treated substrate is then heated in an oven to dry the sensitizer coating and form a thin solid film of the sensitizer on the plastic surface, preparatory to the electroless deposition process.
As described in detail in the copending application, the electroless deposition process involves exposing selected portions of the dried sensitizer film to ultraviolet light through a mask, to form catalytic metal deposition sites at the exposed areas; removing the sensitizer film from the unexposed areas; and then electrolessly plating a conductive metal such as copper on the exposed portions of the substrate surface to form the desired printed circuit pattern.
Another specific example of a process for depositing a metal on a substrate is disclosed in U.S. Pat. No. 4,098,922 herein incorporated by reference which describes a sensitizer liquid with which the present invention is well suited to use.
If the metal deposited on the substrate is to be of a uniform thickness and to adhere properly to the exposed areas of the sensitizer film defining the patterns, the sensitizer liquid must initially be applied uniformly over the surface of the substrate, prior to the drying stage, so as to provide a dried film having a predetermined thickness as uniform as possible.
Additionally, a problem arises with substrates having plated apertures, or thru-holes; that is, holes through the substrate, the walls of which are to be plated such that circuitry on one surface of the board is electrically connected to circuitry on the opposite surface of the board. In this situation, the sensitizer liquid must be applied so that it flows through the holes and forms a uniform continuous coating along the sides of the hole. In this process, there is a tendency for the sensitizer liquid to form a thin skin or "windowpane" across the diameter of the hole, in which case the sensitizer liquid does not flow completely through the hole and uniformly coat the walls thereof. Also, during the subsequent process step of exposing the dried sensitized surface to ultraviolet light, the windowpane tends to prevent the ultraviolet light from entering the hole and forming catalytic sites on the coated walls of the hole. Furthermore, it is desired to apply the sensitizer liquid in a dripless manner, which does not permit any buildup or ridges of sensitizer fluid along edges of the board, particularly along the bottom edge when a simple dip-coating process is used, as described in the copending application.